Liquid crystal displays (LCDs) are advantageous in being light, thin and low in power consumption, and have been widely used in modern information devices such as notebook computers, mobile phones, and personal digital assistants (PDAs). However, a liquid crystal molecule in a general LCD has different refractive indices along a long axis and a short axis thereof. Thus when an LCD is viewed from different directions, images displayed thereon will have a lower contrast as the angle between a sight line of user and a direction orthogonal to the display surface increases, yielding an insufficient view angle in comparison with conventional CRT displays.
Various solutions, for example, In-Plane Switching (IPS) and Fringe Field Switching (FFS), have been proposed to solve the problem of narrow view angle for liquid crystal displays. FFS differs from IPS in that a transparent conductive layer is employed as a common electrode and is entirely placed under a pixel electrode, which gives LCDs using FFS technique a denser distribution of electric field and a larger transmissive area.
FIG. 1 shows a plan view of a pixel structure in a conventional FFS type LCD. For the sake of clarity, FIG. 1 only shows the array substrate portion in the pixel structure, whereas liquid crystal layer and color filters in the LCD, the positions of which are shown in FIG. 2, are omitted. In FIG. 1, reference numbers 101, 102 indicate a data line and a scanning line, respectively, and pixels are formed in the array substrate at crossing areas of a plurality of data lines and a plurality of scanning lines. Each of the pixels has a pixel electrode 106 made of a transparent conductive material and having a plurality of stripe-like slits 107 formed thereon for the transmission of electric field. Operation of each pixel is controlled by a Thin Film Transistor (TFT) having a source electrode connected to the data line 101, a gate electrode connected to the scanning line 102, and a drain electrode 104 connected to the pixel electrode 106 via a through-hole 105. Each pixel also has a common electrode 108 made of a transparent conductive material, which is supplied with voltage via a common electrode bus line 109.
FIG. 2 shows a sectional view taken along a line II-II of the pixel structure of the conventional FFS type LCD in FIG. 1. In FIG. 2, a reference number 210 is used for a glass substrate of a color filter, 211 for a color filter film, 212 for a protection layer, 213 for a liquid crystal layer, 216 for a glass substrate of the array substrate, 215 for a gate electrode insulating layer, and 214 for a passivation layer. Such a pixel structure can be equivalent to a storage capacitor 217 formed between the pixel electrode 106 and the common electrode 108, wherein the pixel electrode 106 and the common electrode 108 serve as the two electrode plates of the storage capacitor 217, and the passivation layer 214 and the gate electrode insulating layer 215 serve as the dielectric between the electrode plates. Alternatively, any material known in the art can be used for the substrates 210 and 216 to replace glass.
In the structure described above, a feed through voltage ΔVp of a pixel, which is the difference of the input data voltage that will pass to the pixel electrode via the TFT and the pixel electrode holding voltage, is represented by the following equation (1).ΔVp=Cgson/(Cgson+CLc+Cst)*(Vgh−Vgl)  (1)Wherein Cgson represents the capacitance value of a capacitor (referred to as a first capacitor) between the scanning line 102 and the drain electrode 104, CLc represents the value of the liquid crystal capacitance, Cst represents the capacitance value of the storage capacitor 217, Vgh represents a high voltage applied on the scanning line 102, and Vgl represents a low voltage applied on the scanning line 102. According to the equation (1), magnitude of the feed through voltage ΔVp is determined by the relation among the respective capacitance values when the high and low voltages on the scanning line 102 is given.
Exposure and etching are often used in manufacturing array substrates for LCDs. During photo processing, etching processing and the like, dimensional deviation may occur in the pixel electrode 106 having the slits 107 due to misalignment in exposed position and unevenness in etching, and such a deviation may even be 1 micrometer or more. Because the pixel electrode 106 serves as an electrode plate of the storage capacitor 217, the capacitance value Cst of the storage capacitor 217 may also vary accordingly, and the feed through voltage ΔVp will also vary according to the above equation (1).
Table 1 shows simulation data of variation in feed through voltage ΔVp for a 32-inch LCD due to the dimensional deviation in the pixel electrode, wherein the high voltage Vgh and the low voltage Vgl on the scanning line 102 are +23V and −6V respectively. It is noted that in the equation (1), Cgson is much less than CLc and Cst in magnitude, and thus the equation (1) may be rewritten into the following equation (2).ΔVp˜Cgson/(CLc+Cst)*(Vgh−Vgl)  (2)In the equation (2), the magnitude of CLc depends on the status of the liquid crystal and can be disregarded. Therefore, the magnitude of ΔVp is substantially reflected by the ratio of Cgson/Cst.
From Table 1 below, it is apparent that in accordance with dimensional deviation occurring in the pixel electrode (that is, non-zero values in the first row of the table), the ratio of Cgson/Cst varies greatly in comparison with the case where there is no such dimensional deviation (the column in which the first row has a zero value). This, in turn, induces substantial variation in the feed through voltage up to more than 25%. Variation in the feed through voltage ΔVp will undesirably introduce flicker of pictures and the like, and cause degradation in the quality of displayed pictures.
TABLE 1variation in feed through voltage for a 32″ LCDDimensional deviation inpixel electrode (μm)−0.5−0.2500.250.5Cgson/Cst0.035030.031120.028010.025500.02343ΔVp (V)1.020.900.810.740.68Variation in ΔVp25.0811.130.008.9316.35(%)